Astrophysics/Cosmology question - redshift

Our universe is observed to be flat, with density parameters Ωm,0 = 0.3 in non-relativistic matter and Ω[itex]\Lambda[/itex],0 = 0.7 in dark energy at the present time. Neglect the contribution from relativistic matter.

At what redshift did the expansion of the universe start to change from deceleration from acceleration?

First I thought about setting (da/dt)*(1/a) equal to zero and solving the equation for z. However, this gives a redshift of -2.32 which doesn't really make sense as the negative value implies that there is a blueshift (right?)

Upon further thought, I realized that a = 0 means that the lefthand side of the equation should be undefined / go to infinity... so could z be infinity? This doesn't really make sense to me either but it's all I've got.

I can manipulate the other equation I found to also give z=infinity by setting a(t) equal to zero.

I'm sure I'm forgetting some important concept but that's what I've got so far.